We are using laser-excitation, europium fluorescence spectroscopy (LEEF) as a tool to probe the reasons behind the preferential binding behavior of peptides for calcium vs. magnesium. The sharp lines present in the excitation spectroscopy of europium bound in place of calcium are diagnostic of the presence of multiple species, or binding configurations and can often be used to determine the extent of hydration of a calcium binding site, the rigidity of the site and estimate how well the calcium ion fits the binding pocket. Peptides which mimic calcium-binding in proteins, such as the EF hand sites in parvalbumin and troponin-C are being investigated.